The invention relates to a process for compressing a gas stream of entry temperature t1 with the help of hydraulically driven piston compressors in two compression stages from an entry pressure p1 to an intermediate pressure p2 after the first compression stage and from intermediate pressure p2 to an exit pressure p3 after the second compression stage, wherein the gas stream compressed to intermediate pressure p2 is cooled back down to entry temperature t1 before entry into the second compression stage and identical pressure ratios, p3/p2=p2/p1, are used in the compression stages.
The invention further relates to a compressor module for performing the process according to the invention with a two-stage compressor part, a drive part, and a power transfer between the compressor part and the drive part over lines with hydraulic fluid.
Piston compressors according to the prior art, which, for example, compress from 1 bar to 300 bars, are built with 3 or 4 stages and driven by a common piston shaft. With a three-stage machine and cooling between the stages, a stage pressure ratio of 6.7 is selected and compression is done from 1 bar in the first stage to 6.7 bars in the second stage to 44.9 bars and in the third stage to 300 bars. The entry pressure can be varied only within very narrow ranges. This is a drawback if the entry gas is made available from a pipeline with 7 bars of pipeline pressure instead of from a gasometer. Another compressor is used that operates at a stage pressure ratio of 3.5 bars.
Thus the object of the invention is to indicate a process and a compressor module for compressing a gas stream that guarantees that a definite, for example constant, final pressure will be reached even at widely varying starting pressure of the available gas, and the same machines are used economically from an energy viewpoint.
This object is achieved according to the invention by a process with the features of claim 1 and by a compressor module with the features of claim 6. Embodiments of the invention are the object of subclaims.
The distinguishing feature of the process according to the invention is that the pressure ratios are adjusted by using the help of two adjustable hydraulic oil pumps accordingly to match, with respect to their flow rate, a hydraulic stream for driving the first compression stage and a hydraulic stream for driving the second compression stage. At an entry pressure that changes from 1 bar to 7 bars, the hydraulic oil stream for the first stage is reduced and the oil stream for the second stage is increased until both stages are operated with the same pressure ratio which, assuming an ideal gas to compress, is the most economical from an energy viewpoint. Because of deviations of the properties of real gases from the ideal gas, and with incomplete cooling down to t1, it can be useful, at almost the same stage pressure ratios, to seek still a more economical operation from an energy viewpoint by experimenting with slight changes in the pressure ratios of the stages. This post-optimization is indeed well known to one skilled in the art, but it can be performed especially easily with the help of the drive according to the invention.
In one configuration of the process according to the invention, the gas stream to be compressed can contain methane or hydrogen or a mixture of methane and hydrogen.
The gas stream to be compressed can contain, for example, a natural gas or a methane-containing fraction of a natural gas.
A variable pressure between 1 and 10 bars can be used as entry pressure p1. In this pressure range, the gas stream to be compressed is almost always made available per pipeline.
A stationary pressure between 250 and 350 bars can be used as exit pressure p3. This is a favorable precondition for filling a pressure tank, a pressure gas bottle or temporary storage.
A distinguishing feature of the compressor module according to the invention is that the drive part for each compressor stage contains a hydraulic fluid pump, each with an adjusting device for the flow rate of the hydraulic fluid. Separate adjustment of the flow rate makes it possible to set the same pressure ratio or postoptimized stage pressure ratios (see above) in both stages and, at the exit of the second stage, precisely the necessary final pressure of the gas to be compressed.
In one configuration of the compressor module according to the invention, the compressor stages can be provided with one fluid-cooled piston compressor each and one aftercooler each. This makes it possible to have an almost isothermic compression and to set approximately the same entry temperature in both compressor stages. This leads to a lower specific compressor power. The aftercooler of the second compressor stage simplifies the filling of a containers subsequent to the compression without heating the latter too greatly.
Every piston compressor can be provided with two working cylinders. The pulsations in the pressure-carrying lines are then especially low.
Cylinder bearing surfaces of the working cylinders can be supplied from without and from within with hydraulic fluid. Cooling is then especially effective.
The hydraulic fluid lines can lead to at least one air-cooled aftercooling device for the hydraulic fluid. One like this has an especially simple design and represents no additional concerns about noise when operated without ventilators, i.e., with natural convection.
The process according to the invention can be used with at least one of the compressor modules according to the invention in a natural gas filling station. A surface-covering entry from natural gas filling stations is favored especially by the fact that, with the invention, the gas to be compressed, in this case the gaseous fuel for vehicles, is withdrawn from pipelines operating at varying pressure and still, with the help of piston compressors of the same type and size, can be compressed.
It can be necessary that the fuel to be compressed must be cleansed of particles and dried in advance. After compression an intermediate storage vessel is useful, from which vehicles can then be filled.